It has been known for a long time, for example from Application EP 231 689, that it is possible to prepare (meth)acrylic anhydride by reaction of acetic anhydride with (meth)acrylic acid in the presence of polymerization inhibitors. The acetic acid formed is distilled during the reaction and the (meth)acrylic anhydride formed is subsequently separated by distillation. However, the use of this process comes up against problems of polymerization during the distillation stage carried out directly from the reactor surmounted by a distillation column. In addition, the amount of (meth)acrylic anhydride produced is limited by the size of the reactor, all the more so as the reactants are introduced all at once into the reactor and as there is, for this reason, no optimization of the reaction capacity.
In order to overcome this problem, the proposal was made, in Application EP 1 273 565, to remove the acetic acid at least in part as it is formed and to at least partially replace the acetic acid removed by continuous introduction into the reaction medium, during the reaction, of acetic anhydride and/or (meth)acrylic acid. This process brings about a substantial improvement in the stabilization of the reaction medium without, however, completely solving the problem of formation of polymers in suspension, which makes it necessary to purify, by filtration, the crude (M)AA2O obtained after removal of the residual reactants and light byproducts. This filtration is problematic to carry out due to the highly lacrymatory nature of (M)AA2O.
Patent Application US 2002/0161260 provides for the use of catalysts based on Cr, Zn, Cu, Ca, Zr, Li, La, Na or Hf in the form in particular of carboxylic acid salts. However, the use of these catalysts does not make it possible to obtain a significant increase in productive output with respect to the abovementioned processes.
The reaction scheme for transanhydrization between (meth)acrylic acid ((M)AA) and acetic anhydride (Ac2O) to produce (meth)acrylic anhydride ((M)AA2O) can be summarized as follows with the two main reactions:
Reaction 1: formation of acetic/(meth)acrylic mixed anhydride and acetic acid (AcOH)H2C═C(R1)COOH+CH3C(O)OC(O)CH3H2C═C(R1)C(O)OC(O)CH3+CH3COOHwith R1=H or CH3.
Reaction 1 is very fast.
Reaction 2: reaction between the mixed anhydride and (M)AAH2C═C(R1)COOH+H2C═C(R1)C(O)OC(O)CH3H2C═C(R1)C(O)OC(O)C(R1)═CH2+CH3COOH
This synthesis is conventionally carried out under batchwise conditions in an installation such as that described in FIG. 1. The configuration for delayed addition of the reactants, such as described in document EP 1 273 565, is not represented in this figure. The reactants, (M)AA and Ac2O, with at least one polymerization inhibitor, are introduced at 1 into the reactor R1 surmounted by a distillation column C1. The shifting of the equilibria is carried out by removing, by distillation at 2, the acetic acid as it is formed. The distillation column C1 acts both to remove the acetic acid generated in the reaction and to shift, for this reason, the reaction equilibria but also to distill, on conclusion of the reaction, the unconverted reactants, the light byproducts of mixed anhydride type and optionally the desired product (M)AA2O. Generally, a distillation fraction F1 composed of acetic acid, a fraction F2 predominantly comprising acetic acid, (meth)acrylic acid and acetic/(meth)acrylic mixed anhydride, a fraction F3 predominantly comprising acetic/(meth)acrylic mixed anhydride with a small amount of (M)AA2O and a fraction F4 predominantly comprising the desired (M)AA2O are collected. The (M)AA2O can also be recovered directly in the reactor heel after distillation of the fractions F1, F2 and F3. However, it is generally necessary to carry out a filtration of this reactor heel in order to obtain a correct quality of the product. The fractions F2 and F3 are for their part generally recycled all at once or continuously to the reaction (not represented in the figure).
The use of the existing processes comes up against problems, more or less marked, of fouling owing to the fact that the reaction is always carried out at high conversion of the reactants. The fact of extending the reaction time in order to achieve a high degree of conversion of the reactants is reflected by an increase in the content of heavy byproducts (Michael adducts type) and the formation of a large amount of (M)AA and (M)AA2O polymers, despite the presence of polymerization inhibitors, at the expense of the selectivity and of the cost of fouling of the reactor.
The fact of carrying out the distillation phase in the same equipment as the reaction further accentuates the phenomenon of production of polymers, which go into fine suspension in the crude reaction mixture. During the distillation phase, the gradual fall in the level of liquid in the reactor is reflected by prolonged contact of the unstabilized or relatively unstabilized monomers, which reflux from the column or which are condensed on contact with cold walls (this is the case, for example, when the dome of the reactor is insufficiently heat insulated). This contacting, by trickling, with the hot wall of the jacket of monomers which are unstabilized or relatively unstabilized is reflected by the formation of polymers which foul the reactor. When the (M)AA2O is recovered in the crude state, freed simply from the fractions F1/F2/F3, it is necessary to filter this in order to remove the suspended polymers. This filtration is rendered problematic by the highly lacrymatory nature of (M)AA2O, particularly during the cleaning of the filters. When (M)AA2O is recovered in the pure state by distillation, the reactor becomes completely fouled up.
Furthermore, the distillation column, generally well suited to the distillation of the acetic acid generated during the reaction, is not always the ideal compromise, both in capacity and in efficiency, for the distillation of the fractions F2/F3/F4. In addition, the presence under hot conditions of the crude reaction product in the reactor throughout the duration of the distillation is very harmful for the reasons described above.